This invention relates to apparatus for automatic temperature compensation of a device of the type effective to generate a voltage as a function of a physical parameter but subject to a predetermined variation of the function with temperature over a predetermined temperature range. Such a device may, for example, be a sensor in which temperature varies the relationship between the output voltage and the sensed parameter or may, as another example, be an electronic signal voltage processing circuit in which temperature varies the relationship between the input and output voltages.
A specific example of a device to which the apparatus of this invention is well adapted is an integrated piezoresistive pressure cell. Such a sensor comprises a bridge arrangement of piezoresistive elements on a pressure responsive silicon diaphragm which produces a differential output voltage representing the sensed pressure. Such pressure cells are well known to be temperature sensitive; and the prior art shows temperature compensation schemes for them.
The typical temperature compensation for such a pressure cell involves compensating the supply voltage across the piezoresistive bridge itself to reduce the temperature dependence of the differential output voltage. This generally involves adding thermistor networks above and/or below the bridge. This has the undesirable effect, however, of reducing the total supply voltage across the bridge itself, with a consequent reduction in the generated differential output voltage. Those skilled in the art of electronic signal processing will recognize that it is desirable in a sensor to maximize the sensor output voltage to increase signal to noise ratio and reduce the need for amplification in the following output signal processing circuit. In the case of a passive sensor such as a piezoresistive bridge, the maximum output voltage is achieved by applying the full supply voltage across the bridge.
In addition, the prior art temperature compensation often involves circuits in a variety of technologies which must be combined on a substrate and which require a large number of adjustments and trims, including, in many cases, high temperature functional adjustments. For example, one known prior art temperature compensation circuit is a hybrid circuit which uses an operational amplifier, special thick film resistors, laser adjustable thick film resistors and a supporting ceramic substrate. Such assembly techniques increase cost and size of the sensor package. In addition, each high temperature functional test requires a specially designed, single use fixture to hold and test the assembled unit at a precisely controlled elevated temperature. This equipment further increases manufacturing cost.